The present invention is directed to a spray desuperheater apparatus and method. More particularly, the present invention is directed to an improved desuperheater valve assembly, which may have a plurality of nozzles affixed to the spray assembly. The aforementioned assembly allows for the controlled injection of a cooling liquid into a superheated fluid to selectively reduce the temperature of the superheated fluid and maintain the downstream flow at a predetermined temperature level.
Superheated fluid is fluid that is at a temperature that is higher than the boiling point of the fluid. Many industrial applications operate most efficiently using a fluid that is saturated or only slightly superheated, but many fluid generators tend to produce a fluid that is excessively superheated. Excessive temperatures (superheat) may damage system components, adversely affect the efficiency of operation or quality of the product being manufactured. The process of injecting a controlled amount of coolant to reduce the temperature of the superheated medium to a specific and precise temperature is defined as desuperheating. Also superheated steam excessive thermal energy can cause damage to the utilizing devices or processes, making it necessary to rigidly control and maintain the steam temperature. Steam desuperheating refers to the process of reducing and controlling the temperature of the superheated steam by introduction of cooling water into the steam flow.
A common method to desuperheat steam is by spraying or injecting a cooling liquid into the flow of superheated steam while it is passing through a steam pipe or the like. Once the cooling liquid is sprayed into the superheated steam flow, water droplets are formed. The droplets rapidly mix with the superheated steam and evaporates, drawing thermal energy away from the steam and thus regulating its temperature. Droplet sizes and spray pattern are among the main parameters, which determine desuperheating efficiency. Thus it is important to utilize a spray nozzle that allows for the injection of a cooling fluid into processed steam or compressible gas with smallest available droplets, optimum spray pattern, and additional mixing control capability.
Because the amount of superheat varies with the amount of steam production and the rate of flow, among other variables, the only true regulation can be made by sensing the temperature of the steam at a point downstream where the injected water has been completely vaporized and heated so that equilibrium conditions have been reached. Conventionally this would require a spray nozzle that is optimized for a very narrow coolant flow rate, and if the flow rate is varied outside of certain parameters, the resulting spray pattern may not give prompt enough heat transfer to allow an equilibrium condition to be sensed and proper steam conditions be attained. However, spring-loaded nozzles allow to eliminate this problem.
An important aspect of desuperheating steam or any gas, is a valve's ability to control the flow of cooling liquid being injected into the superheated steam or any gas. The main control element of a valve is a trim, which usually comprised of three components, a plug, cage, and a seat. In a plug throttling trim types, the plug has a contour on its lower end, below the seat. The contoured portion provides varying flow area with lift, thus regulating the flow. The flow characteristic of the trim varies according to the profile of the plug. Thus, having control valve as a separate unit adds installation and maintenance cost for a desuperheater user.
Due to the service locations, there are more strict requirements for leak tightness for desuperheater coolant control valves in the closed position. If the valve were to leak when the desuperheating was not needed, the unrequited cooling of the steam may cause damage or impede the performance of equipment using the steam. The ANSI/FCI 70-2 (American National Standards Institute/Fluid Controls Institute) standard establishes a series of seat leakage classes for control valves. Class V is usually specified for the desuperheater coolant control valves. It is the most restrictive leakage class for metal-to-metal trim design.
A power positioning actuator is required for kipping the valve fully closed and moving the valve closure membrane, plug, to any position in response to a signal of a temperature control system. The flow over the seat with unbalanced plug design utilizes the valve inlet pressure to assist closure of the valve, which significantly reduces required force from the actuator to provide tight seat leakage shutoff.
To effect movement of the aforementioned plug, which is located inside a valve, there needs to be a transfer of motion from a power positioning actuator to the valve's plug through the body of the valve. The valve may contain liquid at a high pressure and the having a hole in the valve's body to transfer motion to the plug via an actuator may enable leaking at the place where the actuator goes through the body of the valve. One way to prevent leaks is to employ a stuffing box. A stuffing box maybe a gland seal or the like, used to seal a reciprocating shaft against a fluid.
Thus, it would be desirable to integrate the cooling liquid control valve into the desuperheater apparatus to reduce costs by eliminating the need of a separate cooling liquid control valve and its corresponding conduit assemblies. It would also be desirable to have a control valve, which meets the ANSI/FCI 70-2 class V classification characteristics.